1. Field of the Invention
This invention pertains to the manufacture of sintered, ceramic bodies having oriented, controlled porosity. Particularly, multi-layered ceramic bodies, having oriented porosity, on a micron scale, are provided.
2. Background of the Prior Art
Porous, sintered ceramic bodies are used in a variety of applications. Significant applications include insulating bodies, heat exchange devices, filters for high temperature applications, etc. The dominant commercial method for the manufacture of such materials is the extrusion of a ceramic greenstock, through a die with small blockages, which correspond to holes formed in the greenstock. The greenstock is subsequently fired to sinter the ceramic material. Using this process, it is difficult to control the exact orientation, diameter and ultimate form of the pores in the ceramic. Additionally, the extruded greenstock is susceptible to extensive deformation or molding, and must be directly sintered into a ceramic body. Finally, extremely small, micron size pores are not achievable using this process.
Alternative processes are known in the art. Thus, U.S. Pat. No. 2,875,501 discloses the use of heat destructible fiber cores, which, upon sintering, define the oriented porosity in the ceramic body. In this patent, yarns preferably of nylon are drawn through a liquid coagulating agent, which may include a PVA suspension, and then subsequently passed through a dispersion of ceramic precursor, which may be barium titanate. The PVA coated on the nylon yarn adheres the titanate particles to the fiber. A plurality of yarns similarly passed through the ceramic precursor are assembled, and the mass is subsequently fired, to sinter the ceramic. In the process, the fiber, and the PVA are destroyed. The result is a ceramic body having longitudinally oriented continuous holes, or passages, which makes an ideal insulator for conductive material placed in the passages. U.S. Pat. Nos. 2,919,483 and 3,112,184 disclose similar processes, the '184 patent using a carrier sheet, which may be deformed. A common problem to the processes of these patents is the fact that the material, prior to sintering, has no inherent strength, and cannot be molded, handled, or otherwise easily assembled. While the carrier sheet of U.S. Pat. No. 3,112,184 provides for some flexibility, and provides an opportunity to make multi-layered articles, the film itself is the fugitive carrier, and does not permit the formation of small pores, particularly pores in the micron range.
A similar process is addressed by Japanese Patent Publication 297762/48 which teaches the use of a carbon fiber provided with a small amount of binder, such as PVA, dredged in a ceramic raw material powder, which is subsequently fired to form a porous ceramic plate. Alternative processes include premolding a ceramic powder, provided with cavities, and filling the cavities with a graphite powder product, the premolded material being subsequently compressed and sintered, whereupon the graphite is destroyed, resulting in cavities in the ceramic.
Taken as a whole, the art fails to teach a process whereby a ceramic body may be produced, with continuous, small dimension pores, which can be molded into a variety of shapes, or combined with a plurality of layers, in any desired orientation. This is particularly due to the fact that those methods that employ carriers for the ceramic powder, which are subsequently destroyed upon sintering, merely adhere the ceramic powder to the carrier via the use of a coagulating agent, such as PVA, which does not give the resulting, unfired precursor any integrity or body strength.